1. Field of the Invention
The present invention relates to a casting mold for use in continuous casting, which is a so-called casting mold for use in continuous casting of steel in which a molten steel passed through a converter from a smelting furnace is made into a steel. The casting mold of the present invention is made from copper or an copper alloy, whose surface in contact with the molten steel is partly or wholly covered with a coating of a hard metal having not only a high wearing resistance but also a high corrosion resistance i.e., an alloy containing nickel or cobalt, or both together with tungsten, provided that a part of tungsten is incorporated in the form of tungsten carbide.
2. Description of the Prior Art
Conventionally, when a casting mold f or use in continuous casting made from copper or an copper alloy was used as it in the casting, there occurred not only the wear loss of copper, but also damages attributed to the direct collision and the like. Moreover, there was, a disadvantage of forming surface defects denoted as xe2x80x9cstar cracksxe2x80x9d on the bottom of the casting mold due to the scratches that were formed by the direct contact with the surface-solidified ingots. In the light of such circumstances, in order to prolong the life of the casting mold by preventing damages, or to improve the quality of the ingot, there were proposed, for instance, providing a nickel plating or a nickel-iron alloy plating, or providing a three-layered coating comprising nickel-phosphorus alloy layer and a chrome plating on nickel, on the plane, with which the steel melt contacts, of the casting mold made of copper or copper alloy (see JP-B-Sho52-50734, the term xe2x80x9cJP-Bxe2x80x9d as referred herein signifies xe2x80x9can examined published Japanese Patent Applicationxe2x80x9d). Although these proposals were effective to some extent, however, with changing casting conditions, such as an increase in speed of drawing the ingots during casting, an improvement in the powder for use in casting, or the application of an electromagnetic induction stirring, etc., there newly occurred other problems such as the shortening of life due to the problematic generation of corrosion at the bottom portion of the casting mold and due to the increase in the casting speed. Thus, at present, coating materials for casting molds having longer life and further improved in resistances against heat and corrosion are demanded.
As a casting mold meeting such demands, there is proposed a casting mold covered with a thermal sprayed self-fluxing nickel-chromium alloy (see JP-B-Sho60-39453 and 20 JP-B-Sho61-15782). However, in order to achieve high strength and excellent adhesion of the coating with the base material, they require a high temperature thermal treatment to be performed at a temperature as high as 1,000xc2x0 C. in an oxidation preventive atmosphere, and this further requires the use of a precipitation hardened type copper as the copper material to be used for the casting mold. Still, however, the problems of causing degradation and the deformation on the copper material are unavoidable. Furthermore, there are problems of peeling off or of accidents attributed to the dropping of the thermal sprayed coatings. Accordingly, in practice, the application of the methods above is limited to the short edge (narrow plane) of the casting mold for slabs having a smaller area. Furthermore, although there is used a corrosion-resistant material such as nickel-chromium, there still are found corrosion troubles in part of the casting molds.
As a method other than thermal spraying, many proposals have long been made on the so-called composite plating, in which various types of ceramics having lubricating properties and high hardness, such as boron nitride, molybdenum sulfide, silicon carbide, alumina, etc., are dispersed and co-precipitated in a matrix of nickel, cobalt, a nickel-phosphorus alloy or a cobalt-phosphorus alloy, or a nickel-boron alloy, etc. For instance, representative examples can be found in JP-B-Sho58-41933, JP-B-Sho58-23822, JP-B-Sho58-41934, JP-B-Sho58-25534, JP-B-Sho58-41936. In practice, however, those known as dispersion or composite plating tend to cause permanent corrosion on the matrix side due to not only the insufficient junction of the dispersed material and the matrix, but also the difference in potential between the metal of the matrix and the dispersed material. Accordingly, even if the dispersion or the composite plating should exhibit excellent lubricity or wear resistance under the ordinary conditions, it suffers corrosion troubles in the early stage of their practical use due to the specific casting atmosphere, and it cannot fully accomplish its life.
Furthermore, in JP-A-Sho58-212840 (the term xe2x80x9cJP-Axe2x80x9d as referred herein signifies xe2x80x9can unexamined published Japanese Patent Applicationxe2x80x9d) is proposed a casting mold for use in casting. which is covered with tungsten alloydized with nickel. However, on following the process disclosed therein, it was found that the electroplating solution or liquid undergoes decomposition and degradation in a very short period of time, and in case of applying this casting mold in continuous casting, it was found that not only the quality of the alloy cannot be maintained, but the solution or liquid must be newly prepared. Thus, this led to a great waste of economically expensive tungsten salts and nickel salts. Further, the method disclosed in JP-A-Hei7-310196, the plating method comprising dispersing silicon carbide as disclosed in JP-B-Hei4-38838, or the methods disclosed in JP-B-Hei3-69995 or JP-A-Sho60-135592, etc., had the same great problems concerning the stability of the electroplating solution or liquid, and in fact, there is known no practical application thereof to casting mold inclusive of continuous casting.
An object of the present invention is to provide a novel continuous casting mold (i.e. a novel casting mold for use in continuous casting), which can accomplish the requirements for improved life and heat resistance in comparison with any type of recent casting mold for continuous casting, and which can completely prevent the frequently occurring corrosion on the lower portion of the casting mold. Thus, the present inventors have paid attention to an alloy based on tungsten, which had been conventionally considered to possess resistances against heat and wear, but which was never brought into practical use as a coating material for a continuous casting mold, due to the instability of the electroplating solution. As a result of intensive study by the present inventors, it is enabled to solve the aforementioned problems of the instability of the electroplating solution, and as an additional effect, and it is enabled to provide a casting mold for use in continuous casting having longer life, having high resistance against scratches, and yet having excellent heat resistance and capable of realizing high corrosion resistance under the casting atmosphere.
The present invention relates to:
(i) A casting mold for use in continuous casting of steel made from copper or a copper alloy, wherein said casting mold comprises a surface in contact with molten steel partly or wholly covered with a tungsten alloy plating containing either or both of nickel and cobalt, and said plating further containing tungsten carbide in the tungsten alloy.
(ii) A casting mold for use in continuous casting as described in (i), wherein the alloy contains from 0 to 60% by weight of cobalt and from 1 to 40% by weight of tungsten as the components of an alloy with nickel, and the plating is provided at a thickness of from 0.10 to 2.00 mm.
(iii) A casting mold for use in continuous casting as described in (i) or (ii), wherein a nickel or a nickel-iron alloy coating is provided between said alloy plating as described in (i) and the casting mold body made from copper or a copper alloy.
(iv) A method for producing a casting mold for use in continuous casting of steel made from copper or a copper alloy, comprising a surface in contact with molten steel partly or wholly covered with a tungsten alloy plating containing either or both of nickel and cobalt, and said plating further containing tungsten carbide in the tungsten alloy, which comprises electroplating, either partly or wholly, the surface of said casting mold to be brought into contact with a molten steel in a plating solution containing either or both of a nickel salt and a cobalt salt and a tungstate, together with at least one selected from an oxycarboxylic acid and salts thereof, and at least one type of an organic compound and a salt thereof having two or less of carbon atoms within the molecule, provided that its oxidation decomposition potential is lower than that of the oxycarboxylic acid or a salt thereof.
(v) A method for producing a casting mold for use in continuous casting as described in (iv), wherein the oxycarboxylic acid is citric acid, tartaric acid, or an ammonium salt, a sodium salt, or a potassium salt thereof; and the organic compound and a salt thereof having two or less of carbon atoms within the molecule, which is potentially lower than that of the oxycarboxylic acid, is selected from the group consisting of methanol, formaldehyde, formic acid, ammonium formate, nickel formate, sodium formate, oxalic acid, ammonium oxalate, potassium oxalate, etc., and
(vi) A method for producing a casting mold for use in continuous casting as described in (iv) or in (v), wherein the electroplating solution contains an oxycarboxylic acid or a salt thereof at a concentration of 0.5 times or higher of the molar concentration of the total concentration of nickel, cobalt, and tungsten, and contains the organic compound having two or less carbon atoms within the molecule or a salt thereof at a molar concentration of 0.1 to 3.0.